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Month: November 2015

There’s just something so visually pleasing about seeing the orange glow from vintage nixie tubes. I’ve seen commercially available nixie clocks, and even kits, but they cheaper ones I’ve found are still a couple hundred dollars, so I decided I’d make my own. I’d like to start out by talking about why I work on my various projects. Each project I work on, starts with wanting to know more about a part or using a new type of technology. My pocket amplifier for example, I made because I had a need to increase the power from my iPod, but I also wanted to learn how to make my own circuit boards at home. This clock was my first experience with the ESP8266, and the whole reason I made this clock, was to be able to experiment with this board and scratch the surface at what it can do.

A few years ago I bought a VFD clock from Adafruit, but after a couple months the clock would be a few minutes slow. The reason for the drift comes from the crystal oscillator used to keep track of time, even with an accurate crystal, it wasn’t 100% accurate and the error would build each day, until it was a few minutes slow and needed to be reset. For my clock, I wanted to make sure that never happened, and that’s why I decided to use the ESP8266. The WiFi board will talk to a Network Time Protocol (NTP) server and retrieve the current time every day.

The Main Guts

Main Board SchematicNixie Board Schematic

The clock is controlled using an ATmega328, whose only task is to grab information from the ESP8266 and control the nixies. After the ESP connects to WiFi and sends the time, the mega controls the nixies via the HV5530, which is a 32-channel shift register. These chips aren’t cheap, but I much preferred to use a part that’s still in production, than using the vintage 74141, which I’ve only been able to find on Ebay. These shift registers are the same as any other serial in shift register, it takes in serial data and outputs parallel when the latch is enabled. The main difference though, is each pin has an N-channel MOSFET on the output, which can leave each of the nixie’s pins floating at 180V, or grounds them. Given that the output is controlled by MOSFETs, and I was switching 180V, my main concern was the supply voltage to switch the FETs. At 3.3V, the Rds might still be a too high, and cause the FETs to have to waste more power than if they were driven with 12V logic. So in order drive the FETs more efficiently, I incorporated five level shifters to convert the 3.3V logic to 12V logic.

The power supply for the nixies, is just a simple boost converter that takes 12V and boost it up to the 180V to supply power to the anode of the nixies. The inductor uses doesn’t need to be that large, just have a saturation current greater than 100mA. To keep size down, I opted to only display one digit at a time, but I’m flashing through fast enough, that they appear to all be on at the same time.

The ESP8266 Code

The ESP8266 has the responsibility of talking to the internet to fetch the time and update the time, simple right? Well yes and no. It was actually easier than I thought to get the time, using the NTP Client example below.

Using this code, I was successfully able to get the time from the NTP server. I made some changes to send the time over serial to my ATmega328, which reads the time, and displays it on the nixies. The big modification, was adding in HTML code to generate a user interface, to configure the network settings, NTP server address, and changing the time, which comes in as GMT, to my local time zone. These settings are saved on ESP and automatically read on power up.

The clock resets every night at midnight, which I did by using a system restart command on the ESP and for the mega, I just enabled the watch dog timer and would enter a endless loop without petting the dog to trigger a restart.

Lastly, the code used on the mega isn’t in the friendly Arduino style. The main reason was I’m starting to move away from Arduino, and trying to program in straight C. The only Arduino class I kept was Serial. C is just a much more efficient language, and therefore faster, this way I was able to maximize my refresh rate on my nixie display.

Lately at work and at home, I’ve spent a lot of time working with the ESP8266. For anyone that’s familiar, it’s a low cost (as low as 3 USD) microcontroller from Espressif with on board 802.11 b/g/n Wi-Fi, with over 10 GPIO, UART, a single ADC, and SPI. Now that it’s programmable in the Arduino IDE, it’s both insanely cheap, and equally easy to program (writing the software at least).

I’ve been working with mainly the ESP-01, for about $3, you only get a UART and two other GPIO, not great, but for the price, it’s still hard to complain. One thing that is worth complaining about though, is wiring the board to program. With my Arduinos, I use an FTDI programmer, which I can switch between 3.3V and 5V logic. However, with the ESP I’m not able to just use the FTDI board. The issue lies with the power; the ESP is just too power hungry. During normal operation, it can draw up to 250 mA, but I typically see about half of that in steady-state.

With USB 2.0, part of the standard is for the device (in this case the FTDI chip) to negotiate with the host (your computer) for power. If it doesn’t negotiate, the computer will only supply up to 100 mA to that device, this is a problem when the ESP draws close to that when it’s being programmed, and the FTDI uses an LDO to convert 5V down to 3.3V. This means that you have to use an external power supply. So I went out to make a programmer that can program the board and supply power from the USB port. Here’s what I ended up with:

ESP-01 Programmer FrontESP-01 Programmer Back

It’s essentially exactly the same as your standard FTDI breakout board, but instead of using the internal 3.3V LDO, I opted to use a 3.3V buck regulator to increase efficiency and program without hassle, and I also included two buttons:

PROG. – Connected to GPIO_0, and shorts to ground when pressed. On power up, this places the board in programming mode.

RESET – Removes power from the ESP while pressed.

So by pressing and holding PROG. and then momentarily pressing restart, you can boot the board to programming mode. Further more, to make prototyping easier, I broke out power, and the GPIO pins to a header so you can do all of your testing from there, before you deploy the ESP8266.

The full schematic is below along with the design files I made in Eagle, overall the board was under $20 to make, I’ll try and add a bill of materials, but all of my resistors and capacitors are 0603, LEDs are 0805, the inductor for the buck converter was an 0806, and the push buttons are momentary, normally open, 6mm square.